Equity - Stock Repurchases (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 04, 2013
Share repurchases
Total number of shares purchased (in shares)	300,000
Average price paid per share (in dollars per share)	$ 98.98	$ 147.74	$ 113.36	$ 99.00
Total cash utilized	$ 26	$ 1,006	$ 1,097	$ 1,002
Repurchase of treasury shares		$ 1,001	$ 1,123	$ 974
2013 Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program number of shares authorized to be repurchased (shares)					50,000,000
Maximum number of shares authorized for repurchase under stock repurchase plan as percentage of outstanding common stock					18.00%
Share repurchases
Total number of shares purchased (in shares)		6,800,000	9,700,000	10,100,000
Remaining shares available under repurchase program (shares)		19,000,000
Stock Repurchases
Share repurchases
Total cash utilized		$ 1,001	$ 1,097	$ 1,000